1. Field of Invention
This invention relates in general to the construction of the coil for a voice coil motor (VCM), which is used in the transducer positioning system of a magnetic disk file and in particular to an improved coil construction in which the coil is wound from flat ribbon wire.
2. Description of the Prior Art
Magnetic disk files are employed in data processing systems for storing data on the surface of the disk in closely spaced concentric recording tracks. The file includes at least one magnetic transducer, which is moved to a selected recording track by means of a transducer positioning system. In many disk files, the transducer positioning system employs a voice coil motor (VCM) to cause movement of the head and carriage. The movement can involve both a seek mode and track following mode. In the seek mode, the positioning system supplies current to the coil of the VCM to move the transducer from a present track to a target track, in a time optimal fashion. Such movement may involve moving the transducer carriage a minimum of only one track or a few milliinches, in for example, 20 milliseconds. Alternately, the movement may involve a maximum distance of one thousand tracks or approximately 2 inches in an access time of, for example, 40 milliseconds. It will be appreciated that with such movements, the acceleration and deceleration forces on the coil become substantial during a seek operation, since a number of head assemblies are associated with the carriage being moved by the coil. Such an arrangement is shown in U.S. Pat. No. 4,190,870, particularly, FIG. 2, where the VCM is shown moving an assembly having four head arms. FIG. 1 of U.S. Pat. No. 4,144,466 illustrates a similar arrangement in greater detail. In the track following mode, similar forces may also be encountered.
Various coil constructions have been suggested in the prior art for improving the strength of the coil and to improve the overall performance of the VCM. One such construction is shown in U.S. Pat. No. 3,963,882. In other known coil configurations, flat "ribbon" wire is edge wound, such that the flat plane of the wire is substantially normal to the coil axis. "Ribbon" type wire has a substantially rectangular cross-section with an aspect ratio of width to thickness in the range of 3 to 1 to 10 to 1. Since substantial contact surface exists between adjacent turns, bonding these adjacent turns together by use of a suitable bonding agent produces a relatively strong coil. In one prior art arrangement, the coil wire is aluminum covered with a thin layer of polyimide insulation, and a polyimide type bonding agent is applied to the wire during the coil winding process. It has been found, however, that while the strength of the coil is substantially improved in most coils relative to other arrangements, a problem arises in some coils because solvents that are constituents of the bonding agent and by-products which are generated during the coil curing process become entrapped in the turns. The entrapment problem arises primarily because the bond between turns must be relatively thin so as to accommodate as many turns as possible in the space allotted for the coil, and because the depth of the bond, measured in a radial direction relative to the coil axis, is relatively deep. Since the solvent and by-products cannot escape from the coil during the curing process, there is an adverse effect on the coil strength which is not readily detectable until the coil is assembled in the VCM and fails because of poor tensile strength. The present invention provides a coil construction which overcomes the entrapment problem described above.